The present invention relates to the growth of artificial crystals, and more particularly to a method and apparatus for controlling hydrothermal crystal growth to produce crystals with a specific shape.
Hydrothermal crystal growth is the growth of crystals from solution at a high temperature and a high pressure. In a typical commercial process, a vertical autoclave holds a supply of nutrient material immersed in an aqueous solution. An upper portion of the autoclave includes a number of suspended seed plates. The autoclave is heated to increase the temperature and pressure sufficiently to dissolve the nutrient material in the aqueous solution and thereby form a nutrient solution. Typically, the autoclave is raised to a temperature of around 350.degree. C. and a pressure of 10,000 p.s.i. A temperature gradient inside the autoclave creates convective currents, which carry the nutrient solution upward. The nutrient solution then cools and is deposited on the seed plates, thereby causing crystal growth.
Hydrothermal crystal growth is used to grow crystals composed of nutrient materials having very low solubilities in pure water. Some of these materials include quartz (SiO.sub.2), zinc oxide (ZnO), calcite (CaCO.sub.3) and aluminum oxide (Al20.sub.3). Although these materials are more soluble under hydrothermal conditions, mineralizers are typically included in the aqueous solution to achieve reasonable solubilities. In commercial crystal growing, the mineralizers are almost always alkaline (NaOH and Na.sub.2 CO.sub.3 are common choices) but neutral or acidic materials can also be used. The choice of mineralizer depends on the material being grown and the impurities which are acceptable.
The most commercially significant crystals that are grown hydrothermally are quartz crystals. Quartz crystals are commonly used in the electronics industry to manufacture quartz oscillator plates. Quartz crystals are also used in optical spectrographs and other optical devices. After being artificially grown, quartz crystals are lumbered and cut to form quartz wafers. Currently, most of the purchasers of quartz wafers desire quartz wafers having a circular shape with a portion cut away to form a reference flat. The length of a wafer extending perpendicularly from the reference flat to the outer edge of the wafer is often referred to as the segment height of the wafer. Typically, purchasers require the circular quartz wafers to have a diameter of either three inches (3") or one hundred millimeters (100 mm)
In the science of crystallography, the axes of a crystal are normally designated the x, y and z axes, each axis being angularly related to each of the other two axes. A naturally-occurring quartz crystal is elongated and has a generally hexagonal cross-section with pyramidal ends of six facets each. The z axis of the naturally occurring quartz crystal extends longitudinally thereof, while there are three x and three y axes perpendicular to the z axis. The x axes intersect the angles formed by the sides of the crystal, while the y axes are perpendicular to such sides.
In commercial growing processes, crystal growth in the direction of the z-axis is typically preferred over growth in the direction of the y-axis or growth in the direction of the x- axis. In the direction of the y-axis, growth is practically non-existent. In the direction of the x-axis, growth quickly tapers to an edge. Growth in the direction of the z-axis, however, is fast and does not quickly taper to an edge. In addition, growth in the direction of the z-axis results in considerably less impurity incorporation than in the other directions.
Seed crystals have been adapted to take advantage of the preferred growth in the direction of the z-axis. Expired U.S. Pat. No. 3,291,575 to Sawyer, which is incorporated herein by reference, shows a seed plate having its greatest length in the direction of the y-axis and its shortest length in the direction of the z-axis. In this manner, the seed plate has a length in the direction of the y-axis, a width in the direction of the x-axis and a thickness in the direction of the z-axis. Such a seed plate is often referred to as having a z-cut. A z-cut seed plate has a major face disposed substantially perpendicular to the z-axis or substantially parallel to a plane defined by the x and y axes. This major face and its companion major face on the opposite side of the z-cut seed plate are the greatest areas on the z-cut seed plate. In this manner, the z-cut seed plate promotes crystal growth in the preferred direction of the z-axis.
In many prior art commercial growing processes, seed plates are freely suspended in the autoclave. As a result, crystal growth often occurs in undesirable directions, such as in the direction of the x-axis. Crystal growth in such undesirable directions tends to be flawed and produces crystals having a shape and size that is not conducive to efficient commercial utilization.
In order to prevent undesirable crystal growth, some prior art processes suppress crystal growth in the direction of the x-axis using restrictor plates or shields. Examples of such prior art processes include those shown in U.S. Pat. No. 5,069,744 to Borodin et al., U.S. Pat. No. 3,607,108 to Gehres, U.S. Pat. No. 3,013,867 to Sawyer, U.S. Pat. No. 2,674,520 to Sobek, and Sawyer 575', all of which are incorporated herein by reference.
Even if crystal growth in undesirable directions is suppressed in a process by restrictor shields, the crystals that are grown in the process will still have a shape that is not conducive to efficient commercial utilization. The restrictor shields will produce crystals with planar sides and sharp angles. These planar sides and sharp angles will have to be eliminated by a substantial amount of lumbering in order to produce the desired circular wafers.
Based upon the foregoing, there is a need in the art for a method and apparatus for forming crystals having a shape conducive to efficient utilization. The present invention is directed to such a method and apparatus.